1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing fiber gratings, and an optical fiber having the fiber gratings formed thereby, and more specifically, to a method and apparatus for manufacturing polarization-independent type fiber gratings, and an optical fiber having the fiber gratings formed thereby.
2. Discussion of Related Art
Generally, it is observed that an optical fiber has the same shape in a longitudinal direction. Therefore, except for defects caused in the manufacturing process, the optical fiber persists its configuration regardless of its portion. However, a refractive index of a core of an optical fiber may be changed in the longitudinal direction, and portions of the optical fiber having different refractive indices from other portions of the fiber are referred to as fiber gratings. In other words, the fiber gratings are periodic structures of an increased refractive index compared to other portions of a core.
The fiber gratings, one of the most important optical devices, have numerous applications such as a band-pass/reject optical filter, extraction of a clock signal, DPSK demodulation, an optical CDMA encoder and decoder, etc.
Generally, the fiber gratings are manufactured by irradiating UV rays onto an optical fiber 12, as illustrated in FIG. 1. A core of the optical fiber is formed of germanium-doped silica glass. When the UV rays from a light source 16 are irradiated onto the core, atomic binding is broken and the region where the atomic binding is broken has a different refractive index from other regions. Therefore, a mask grating 14 is disposed between the UV-ray light source 16 and the optical fiber 12, and UV rays are irradiated onto the mask grating so that fiber gratings, in which a refractive index is periodically changed, can be obtained.
However, when the fiber grating is manufactured using the above method, birefringence may be caused when light passes through the optical fiber. As disclosed by Ashish M. Vengsarkar, et al. in, “Birefringence Reduction in Side-Written Photoinduced Fiber Devices by a Dual-Exposure Method,” Optics Letters, vol. 19, No. 16, p. 1260, Aug. 15, 1994, birefringence is mainly caused by geometrical asymmetry generated when fiber gratings are formed.
The cause of birefringence generation will be described below with reference to the accompanying drawings. As illustrated in FIG. 1, when it is assumed that a depth direction, toward which the UV ray is irradiated, is plotted on an x axis, and a cross-sectional direction of an optical fiber is plotted on a y axis, which is perpendicular to the x axis, FIG. 2 represents a change in refractive index (Δnx) of the optical fiber versus the direction of the x axis, and FIG. 3 represents a change in refractive index (Δny) of the optical fiber versus the direction of the y axis. As can be seen from FIG. 2, as the core is closer to the light source 16 (i.e., when a value of the x axis is high), a drastic change in refractive index is observed. On the other hand, when the core is further from the light source 16 (i.e., when a value of the x axis is low), a small change in refractive index is observed. Similarly, in FIG. 3, an asymmetrical change in refractive index is shown based on a region where the UV rays are extensively irradiated.
In FIG. 2, an average value of the change in refractive index with respect to the total length of the x axis direction is represented by a dotted line. Also, in FIG. 3, an average value of the change in refractive index with respect to the total length of the y axis direction is represented by a dotted line. The inconsistency between the two average values in an optical fiber, in which the fiber gratings are formed, causes birefringence and polarization dependence, in which light energy loss is changed depending on a polarization status of light.